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RCCB & Switch Overload Protection (536.4.202)

sparkiemike
Just trying to get my head around regulation 536.4.202, particularly the last line.


“However overload protection shall not solely be based on the use of diversity factors of the downstream circuits. To achieve overload protection of RCCBs or switches, the rated current of the OCPD shall be selected according to the manufacturers instructions”.


If I understand it correctly in a the sum of the downstream MCBs should not exceed the current rating of the RCD. So in most dual RCD split load boards the RCDs would need to be rated at 100A


Is this correct, because there seem to be a lot of "pre-populated" boards still being sold with 63A RCDs? 


How are others interpreting this new reg?
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  • 0
    536.4.3.2 includes The rated current of a switch or RCCB may also be based on the application of diversity factors to the downstream

    circuits according to Regulation 311.1 , and the rated current of the OCPD shall be selected according to the

    manufacturer's instructions. See also Regulation 536.4.202.


    Which at first sight contradicts.

    so we need the rest of  536.4.202
    The rated current of a switch or RCCB (InA and Inc) shall be based upon:

    - the sum of final circuit current demand after any applicable load diversity factors, or

    - the sum of final circuit current demand after any applicable load diversity factors together with allowances

    for diversity between final circuits, or

    - the sum of the downstream OCPDs/circuit rated current multiplied by a diversity factor.


    which pretty much says you can have 100A total of final circuits on a 63 A RCD, as diversity will sort it for you. Equally if the house fuse is 100A, then to assume a 63:37 split or better on a two RCD board is probably a reasonable assumption.

    The last clause is really saying before you  this, check with the makers instructions that it is OK.

    I think we can assume that if a ready built board from say Wylex uses one of their own RCDs in this way, the makers are indeed OK with this.

    Well that's my reading of the tea leaves anyway - I agree it would benefit if it was clearer, and terser.

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  • 0
    536.4.3.2 includes The rated current of a switch or RCCB may also be based on the application of diversity factors to the downstream

    circuits according to Regulation 311.1 , and the rated current of the OCPD shall be selected according to the

    manufacturer's instructions. See also Regulation 536.4.202.


    Which at first sight contradicts.

    so we need the rest of  536.4.202
    The rated current of a switch or RCCB (InA and Inc) shall be based upon:

    - the sum of final circuit current demand after any applicable load diversity factors, or

    - the sum of final circuit current demand after any applicable load diversity factors together with allowances

    for diversity between final circuits, or

    - the sum of the downstream OCPDs/circuit rated current multiplied by a diversity factor.


    which pretty much says you can have 100A total of final circuits on a 63 A RCD, as diversity will sort it for you. Equally if the house fuse is 100A, then to assume a 63:37 split or better on a two RCD board is probably a reasonable assumption.

    The last clause is really saying before you  this, check with the makers instructions that it is OK.

    I think we can assume that if a ready built board from say Wylex uses one of their own RCDs in this way, the makers are indeed OK with this.

    Well that's my reading of the tea leaves anyway - I agree it would benefit if it was clearer, and terser.

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